General code domain multiplexing includes multiplexing data in multiple code layers, in which each layer has its own codebook or signature.
HARQ is a retransmission technique in which data may be transmitted with a Forward Error Correction code. Upon reception of erroneous data, a receiver may request retransmission. There are two main techniques used for retransmission, namely Chase-Combining (CC) and Incremental Redundancy (IR).
In Chase-Combining, the originally transmitted data is repeatedly retransmitted along with parity bits. The retransmitted data and parity bits are combined with the originally transmitted data and parity bits using Log Likelihood Ratios (LLR) or symbol level combining. Signal combining is equivalent to power accumulation leading to a better quality of the equivalent channel and higher chance of successful decoding of the original data.
In Incremental Redundancy, new parity bits are transmitted with every retransmission until an acknowledgement (ACK) is received. The new parity bits along with previously transmitted parity bits reduce the equivalent code rate of the data transmission with better error protection capability. A stronger code may lead to the successful decoding of the original data.
Under either CC or IR, the HARQ process is stopped until the original data is successfully decoded and an acknowledgement (ACK) is sent from the receiver to the transmitter, or the HARQ process reaches its maximum number of allowable transmission attempts.